Semiconductor lasers are attractive for a wide variety of applications including telecommunications, computing systems, optical recording systems and optical interconnection of integrated circuits. Semiconductor lasers provide compact sources of coherent, monochromatic light which can be modulated at high bit rates to transmit large mounts of information.
Vertical-cavity surface emitting lasers are particularly promising for applications requiring two dimensional arrays of lasers. As contrasted with edge emitting lasers which emit light parallel to the semiconductor growth planes of their substrates, VCSELs emit light perpendicular to growth planes. A typical VCSEL comprises an active semiconductive region sandwiched between a pair of distributed semiconductive Bragg reflectors. Upon injection of suitable current through the active region, laser light is transmitted perpendicular to the growth planes.
An important feature for VCSELs in laser applications requiring optical fiber coupling or low noise performance is single-mode operation with high output power (.gtoreq.1 mW). Typically VCSELs operate in a single longitudinal mode owing to their short cavity length; however, optical emission in a higher-order transverse mode is common at modest output power, and can be multimode at high power. To achieve and ensure single transverse mode operation, an approximate cavity diameter of .gtoreq.5 .mu.m is desirable if not required.
In a paper entitled "Vertical-Cavity Surface-Emitting Laser Diodes Fabricated by In Situ Dry Etching and Molecular Beam Epitaxial Regrowth" written by Kent D. Choquette et at. and published in IEEE Photonics Technology Letters, Vol. 5, No. 3, pp. 284-287, a method was described for fabricating lasers having mesa-type structures. In that method, a Group III-V semiconductor laser mesa structure is fabricated by first forming a III-V mesa structure on a III-V semiconductor substrate, followed by forming by means of molecular beam epitaxy an epitaxial sidewall layer at a time when the top surface of the III-V mesa structure is protected by a sacrificial layer of silicon dioxide. One of the purposes of the epitaxial sidewall layer is to provide a relatively defect-free sidewall for confining laterally the optical radiation generated in the laser structure.
During the molecular beam epitaxy of the epitaxial sidewall layer, a polycrystalline ("non-epitaxial") layer of the Group III-V semiconductor material being deposited on the sidewall also forms on the top surface of the mesa. This polycrystalline layer and the sacrificial layer of silicon dioxide must be removed in order to enable an ohmic contact layer to be attached to the top surface of the epitaxial mesa structure. The removal of this polycrystalline layer, in turn, requires a separate lateral alignment step to align an aperture in a photoresist layer through which the required removal of the polycrystalline layer (by means of reactive ion etching) is performed. Owing to lateral misalignment errors in forming the aperture in the photoresist layer, it is difficult to achieve mesa diameters of as small as approximately 5 .mu.m using the method described in that paper. Moreover, the electrical resistivity of the epitaxial sidewall layer may be insufficient to prevent an undesirably large amount of power dissipation caused by leakage current to the substrate--i.e., caused by leakage current between a wiring layer (located on the top surface of the structure) and an electrical contact to the substrate.
It would therefore be desirable to have a method for fabricating small-diameter lasers that is more easily controllable and that can, if desired, reduce the leakage current.